Process for the transportation of impellent gases, for example natural gas, and apparatus for carrying out the process

ABSTRACT

A method and a device for transportation of natural gas and other buoyant impellent gases, without the need for expensive pipes and liquid tank cars or ships are described. The natural gas is air-lifted from a starting point to a consignment point by means of voluminous, light, hollow bodies, and said bodies being borne en route to the consignment point by the lift of the impellent gases. Upon release of the gas at the consignment point, the bodies are filled with another buoyant gas for the return trip to the starting point. At the starting point the said buoyant gas is again exchanged for impellent gas which is to be transported, so that in both directions substantial additional quantities of supplementary freight goods can be transported.

United States Patent [1 1 Papst Oct. 29, 1974 [54] PROCESS FOR THE TRANSPORTATION OF 2,731,168 l/l956 Watts 114 74 R IMPELLENT GASES, FOR EXAMPLE g p H s Ol'lS egaar NATURAL AND APPARATUS FOR 3,53l,064 9/1910 McCutchan 244/31 CARRYING OUT THE PROCESS Hermann Ernst Robert Papst, 7742 Saints Georgen Karl-Maier Strasse, Schwarzwald, Germany Mar. 19, 1973 Inventor:

Filed:

Appl. No.: 342,545

Related US. Application Data Continuation of Ser. No. 69,328, Sept. 3, 1970, abandoned.

Foreign Application Priority Data Sept. 9, i969 Germany 1945608 References Cited UNITED STATES PATENTS 8/1928 Naatz 244/30 Primary Examiner-Duane A. Reger Assistant ExaminerGalen L. Barefoot Attorney, Agent, or Firm-Cushman, Darby & Cushman [5 7 ABSTRACT A method and a device for transportation of natural gas and other buoyant impellent gases, without the need for expensive pipes and liquid tank cars or ships are described. The natural gas is air-lifted from a starting point to a consignment point by means of volumi- 5 Claims, 7 Drawing Figures IMPELLENT GASES, FOREXAMPLE NATURAL GAS, AND APPARATUS FOR CARRYING OUT THE PROCESS This is a continuation, of application Ser. No. 69,328 filed Sept. 3, 1970, now abandoned.

BACKGROUND AND BRIEF DESCRIPTION OF INVENTION The invention relates to a process and a transport device for the transportation of impellent gasses, for example natural gas or hydrogen, from a starting point at which the impellent gas is available to a place of consignment at which the impellent gas is to be used.

In making natural gas available for use, there is the substantial difficulty that transportation of the gas from a point at which it is obtained to industrial plants where it is to be used entails, in general, very high investments in terms of pipe lines and pumping stations, which, moreover, are often utilizable for only a few years because many sources of natural gas are relatively rapidly exhausted.

It is known, of course, that natural gas can be liquefied and can be transported by means of deep cooling tanks to the points of consignment or to the central pipe line connections in harbors. Aside from the feet that for this kind of transportation considerable amounts of pipe lines overland to the harbors and from the harbors are required, the liquefying of the gas and the transportation of the liquefied gas involves high expenditure and is potentially dangerous.

For this reason it is not worthwhile to exploit average or smaller sources of natural gas at a distance from'the places of consignment, and thus, natural gas is wasted in the course of petroleum recovery and is uselessly burnt up, although with its large fraction of gaseous alkanes it is of considerable value not only as a fuel but also, for example, for the production of synthetic materials.

The object of the invention therefore is the development of a process and a system for the transportation of impellent gases, particularly natural gas and pure methane gas. or even helium, which avoids the drawbacks of known arrangements, whereby gas of this type can be delivered to points of industrial use from remote locations in an economical and safe way.

The invention involves a process for transporting an impellent gas in voluminous light hollow bodies which are particularly insulated for a slightloss of heat. These bodies are moved through the air; and en route to a point of consignment they are supported by the buoyancy of the impellent gas and can carry along balancing freight or ballast. Upon release of the impellent gas at the place of consignment they are filled with another cheap buoyant gas and freight or ballast which is available everywhere for the return journey to the starting point, where buoyant gas is again exchanged at the starting point for more impellent gas to be transported.

An especially advantageous example of the invention consists in providing the transporting hollow bodies with self-propelling and heat insulated hulls which are, in the return journey to the source of the impellent gas, kept filled with heated gases, for example air and/or steam. For this return transportation of the hollow bodies to the source of impellent gas, hot or heated airwill be the cheapest and the simplest material to use, if the airship is built light enough for this. A preferred embodiment comprises the use of saturated steam and a saturated steam-air mixture as buoyant gas for the return trip. The steam is held in the return journey at a temperature which corresponds to its dew point in that as much fresh steam is delivered as is condensed by the heat loss of the carrying body.

Steam is relatively cheap and is available in industrial centers as a side product that is difficult to utilize (for instance, in steam-electric generator plants). When the body is re-filled at the starting point, the steam can be either condensed and utilized as water or simply let out. For balancing the heat loss by the nonrigid hull, the heat of the exhaust gases of drive engines can be used.

On the journey, steam, corresponding to the partial pressure relationship of the steam-gas mixture, which corresponds to the condensation or dew point, may be used supplementarily to the impellent gas as a buoyant gas. It is especially advantageous because of its high heat of evaporation for stabilizing the temperature of the buoyant gas. Walls of the non-rigid envelope of the airship can be double walled and can be partitioned off by bulkheads to from gas-tight sections. In order to prevent danger of fire from the natural gas charge in the hollow body, the intermediate space between the double walls of the non-rigid hull is held at excess pressure of air or a non-combustible gas, for example nitrogen, carbon dioxide or sulfur hexafluoride (SE Lines for the pressurized gas in the intermediate spaces lead to the separate sections.

Complete security against fire even in an experiment with combustion can be developed in that the combustible impellent gas for the trip can be mixed with a fraction of steam, whereby the combustibility of the impellent gas-steam mixture is reduced or eliminated. For this it can be sufficien't to add to the impellent gas, for example methane, about 27 percent steam; and by the low partial pressure of the steam a temperature of 67 C will be maintained. For recovery of the pure natural gas, the steam addition will be condensed at the point of consignment. With this method it is also possible to transport relatively heavy gases, such as ethane, propane, butane, etc., with the necessary lift being developed by a correspondingly high amount of steam or another buoyant gas in separated chambers within the hollow envelope.

On the other hand, in special cases during transportation, it is possible to use a specific amount of steam, held constantly at the dew point and separated from the impellent gas, for heating the buoyant gas for greater support. Since there is practically no difference in pressure between a chamber filled with natural gas and another chamber filled with steam, suitable separating partitions can be made which are therein and correspondingly light. These partitions can also be double walled.

In order however to meet the problem of heat loss in the transportation of gases in the special conditions of using hot air and steam, an airship of the type described in US. Pat. No. 3,456,903 is recommended. Accordingly, the invention consists also in the use of an airship of this kind as transporting body. Such an airship is an engine driven, semi-rigid dirigible with an inflated double walled, non-rigid envelope for containing a volume of buoyant gas. The double walls of the envelope are filled with a gas, for instance air or CO which is a poor heat conductor, under a pressure that is higher than the maximum pressure of the buoyant gas. Between the inner and the outer wall there are intermediate members that prevent heat convection as much as possible. In addition, there are flat elements extending between the walls, whereby steam as a buoyant gas and heat transfer medium acts within the double walled nonrigid envelope as saturated steam. The intermediate members comprise closely disposed, collapsible traction connections that connect the inner and outer wall practically radially, and the flat elements comprise closely disposed supplementary, collapsible heat reflecting surfaces, which are disposed in a zig-zag between the inner and the outer wall.

A transporting hollow body of this kind allows the carrying out of the process of the invention surprisingly well. With more or less complete expulsion or sucking out of the heat insulating gas from the intermediate space in the double wall, particularly in the upper region of the non-rigid envelope (and also with its invertible intermediate partitions) a pronouncedly increased heat transfer can be effected, in that the walls of the double wall can contact each other, when needed, to thereby pronouncedly accelerate condensation of the steam which is enveloped by the hull. This reduction of the protective gas pressure in the double wall envelope is attained, for example, by means of the lift pressure of the gases enveloped by the hull. The heat insulating gas which is expelled by said gas pressure can be taken up in expanding containers outside or inside the nonrigid envelope.

The special propulsion of this hollow body is primarily suited to the use intended in the present invention. This consists of air jet propulsion means that are provided with compressors driven by low noise level engines disposed on a keel structure of the transporting hollow body. The propulsion means is in the form of a jet nozzle that has an annular gap nozzle aperture about an inset. This arrangement provides for a steering device in that the inset body and the annular gap of the nozzle opening are adjustable by control devices disposed axially and radially with reference to the long axis of the nozzle.

For take-off and landing on terrain that is not specially prepared, the use of a special landing means is also proposed. The airship has a rigid keel structure with a substantially flat rigid bottom, which is furnished on its underside with a hose-like cushion seal that encloses the bottom surface. At least one suction opening of a suction fan is provided in the enclosed bottom. On landing, the airship immediately sucks air from under the keel when the cushion seal approaches the ground surface and is applied to it, so that the intake pressure is transmitted to the ground. This keel frame can also be made floatable so that at sea it can serve as a ship hull. If with this landing means the suction side of the strong propulsion compressors are used for sucking the airship, there is an optimally safe landing and the airship is most reliably anchored even at wind velocities up to the maximum of e.g., I44 km/hour.

In special adaptation of the airship used according to the invention for the transportation of gases, the airship is converted in a particularly advantageous embodiment so that it presents symmetrically disposed and flexible partitions inside the buoying body, which partitions are invertible and divide the inner chamber of the buoying body into separate compartments and the present separate gas intakes and outlets. Thus the capacity of the separate compartments can be controlled, to range practically from zero to the complete capacity of the buoying body by means of the invertible partitions, with filling of one compartment and emptying of the other and vice versa.

To maintain a uniform altitude in flight, or uniform weight of the hollow body, the engines may be driven advantageously about half with propellant fuel and half with impellent gas (methane), so that the buoyancy by the consumption of the propellant fuel is reduced. In operation of the engines with fuel oil alone, the reduction in buoyancy through condensation of steam will advantageously be initiated only toward the end of the flight, so that flight can be at a greater height and hence on the average somewhat faster.

As a special advantage of the method of gas transportation by air through a hollow or a container it is to be noted that both on the way out and on the return substantial amounts of supplementary freight can be carried, e.g., sand, crude oil, and other products (heavy machinery, drill rigs), and also, because of the explosion proof gas charge, passengers. Such transporthollow bodies can take off and land in the most out of the way places and they are independent to a considerable extent of storm conditions, all the more so since the heat insulating non-rigid envelope of this invention reduces the effect or outside temperature on the gas that is being carried.

With the method of the invention not only will there be elimination of the pipelines and liquid gas tankers that have been used so far, but the transportation of impellent gases can also be effected without the loss of time that is entailed in such arrangements. A gas transporting hollow body of this kind can have, for example, a diameter of 106 m and a length of 364 m.

The useful load of natural gas (with percent transport gas filling minus gas for own use) is about 1,900,000 cubic meters of CH C) and the additional free freight capacity is 1,200 t, e.g., for oil. On this basis the transport cost comes to 0.222 German pfennings for each cubic meter natural gas and 1,000 km distance if at the same time l,200 t oil with the same computation of caloric value is transported.

The invention is described below in more detail with reference to various examples as illustrated.-

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 shows a filled hollow body or container with a preferred arrangement of inner partitions (dashed lines) within its hollow envelope;

FIG. 2 shows the hollow body of FIG. 1 when its envelope has been emptied;

FIG. 3 shows the same hollow body in a re-filled condition;

FIG. 4 shows a longitudinal view of the stern of an airship like hollow body;

FIG. 5 shows a schematic cross section through the keel structure with a hose-like cushion seal on the underside;

FIG. 5a shows an enlarged cross section taken from the heat insulating double walled non-rigid envelope of the hollow body as indicated in circle Va of FIG. 5; and

FIG. 6 shows a schematic representation of an airship with various gases in symmetrically filled compartments that are constituted by the invertible partitions.

DETAILED DESCRIPTION OF INVENTION The hollow body or container for the gas transport having a shape like an airship 4 as shown in FIGS. 1 through 3 comprises a transport body having a hollow bow section 1, a hollow, cylindrical mid-portion 2, and a hollow stem part 3. The hollow body is formed as an envelope 40 made up of a double walled construction of the type shown in greater detail in FIG. 5a. The double walled construction includes an inner wall 41 and an outer wall lamination 42, 43, and 44. The inner and outer walls are radially interconnected by traction devices 45 that are spaced about 5 cm apart. Air, or another non-combustible gas, e.g., nitrogen or carbondioxide, as introduced between the innerand outer walls of the envelope at a pressure above the inflation pressure of the buoyant gas contained within the main space within the envelope. This assures a stability of the hollow body 40 during flight, and in addition, provides for a gas-filled intermediate space 13 between the two walls making up the envelope. This space functions as a protective shield around the gas contained in the envelope and as a heat insulating layer around the envelope. In addition to the heat insulating characteristics of the intermediate space 13, the two wall surfaces are provided with thin metallic layers, e.g., 42 that does not limit the flexibility of the envelope and which are both compact and sealed against diffusion. Also, inside the intermediate spaces 13 between the walls, there are metal coated thin foil surfaces fixed at their extremities to the outer and inner walls and which present a plurality of folded reflecting surfaces, e.g., in a zig-zag configuration. Heat radiation is thus reflected so that the heat insulation is improved. These foil surfaces are not interconnected lengthwise, so that a balance of pressure inside the intermediate spaces 13 is not prevented. Heat convection flows are prevented by the close spacing of the individual foil strips and the intermediate strips. The buoyant space inside envelope 40 can be defined with reference to the bow 1 and the stem 3 by flexible walls that are similar to invertible heat insulating walls 7 of the envelope mid-section 2.

The inner sections 3a and 4a of envelope 40 are separated by walls 7 with reference to the stem 3 and the bow 1, respectively, and are in direct connection with the outside air (or they may be connected via auxiliary blowers and possibly via a propulsion compressor) to create a uniform pressure differential. Since walls 7 have an extent that is much greater than the inner cross section of envelope 40, they can, depending upon the difference in pressure between the buoyant gas and the atmosphere, either puff out into the bow or stem part of the envelope (see the double dashed position A of FIG. I) or they can reverse into the propellant gas chamber (see the plain dashed position of FIG. 2). In this way the conically shaped bow and stem sections of envelope 40 can also function as trim and stabilizing cells.

Thin foldable walls 33 are sealed to the places of attachment of walls 7 inside envelope 40. Walls 33 with walls 7 form two separate chambers which can be filled or emptied via connections 15 and 17. As soon as a gas has been admitted via connection 16 and the contents of the chamber enclosed by walls 33 has been drawn out or forced out via connections 15 and 17, walls 33 according to FIGS. 1 and 3 are pressed against walls 7 of the envelope so that practically the entire space inside the envelope can be filled via connection 16.

The hollow body 1-3 according to FIG. 4 is propelled by nozzles 9 on the stern 3 (see FIG. 4). From these nozzles air is driven out, as delivered via passages 5 from engine-driven compressors in keel structure 10. The drive of the compressors can be effected by diesel engines or by natural gas engines. The control of hollow body L3 is effected by the radial and axial displacement of nozzle body 22, with reference to the longitudinal direction of the said hollow body, by means of adjusting members 8 (by known hydraulic devices), so that an annular gap 9 of the nozzle can be suitabley varied. This control effects an adequate maneuverability for the hollow body.

A keel structure 10 is fixed under envelope 40 by belts 31 (see FIG. 5) for providing space for crew and passengers, and space for all freight and whatever machines are accommodated. The keel structure can be made seaworthy so that the airship can come down on the water at sea.

According to FIG. 5 the keel structure 10 is built as a pipe construction 23 whose inner chamber can safely accept liquid fuel. The keel frame has a hose-shaped cushion, or semi-circular sealing protuberance, 24 around at least a part of its bottom surface, with which elements it is set down on the landing surface. Within these sealed bottom surfaces there opens the suction side of a compressor 6 so that the keel frame is sucked firmly against the ground in starting and landing. Instead of a special compressor 6' a compressor 6 of the propulsion means can effect the suction directly to obtain immediate holding.

In the keel structure 10 there are also devices e.g., steam boilers or heat exchangers for creation of steam. The heat of the exhaust gas of the propulsion engines is also utilized for this purpose. Excess is carried off with the nozzle air. On keel structure 10 there are also connections for conduits 15, 16 and 17 at suitable locations for the filling and emptying of the individual compartments for the buoyant gas.

The natural gas that is to be transported can be shipped, as shown in FIG. 1, in the closed mid-section container 14 which is filled and emptied via connections 16. The mid-section chamber of container 14 can be formed directly between the invertible walls 7 and inner walls of envelope 40 or additional thin invertible walls 33.

As shown in FIG. 1, the additional thin separating wall 33 can be adjacent to the wall 7 which can be turned inside out on the side lying toward the cylindrical center part 2, to thereby define between the walls 33 an inner space for the container 14. Adcording to FIG. 3, on the other hand, there can be in each end one more wall 33 provided. Thus, separate chambers can be formed by walls 33, and the separate chambers can be filled according to choice with different gases, such as natural gas and/or hydrogen and/or helium. The filling and emptying of the various chambers by mutual displacement and expansion of the individual chamber contents has been shown schematically in FIG. 6, wherein natural gas is shown at 27.

On both ends container 14 is surrounded by steam 29 or hot air 28 so that if there is a leak of natural gas from container 14 it will only be mixed with the steam. For the transportation process container 14 is filled full with natural gas, at the expense of the surrounding steam-filled space, whereby the steam is either drawn off via connections 15 and 17 or is condensed by cooling, for which the heat insulating intermediate chamber in walls 7 is compressed by removal of its excess pressure by cutting off an idling auxiliary compressor.

If at the place of consignment the natural gas is to be discharged. steam 29 or hot air 28 is let into the buoyant chamber through connections l and 17 and at the same time a corresponding amount of the impellent gas, i.e., natural gas or hydrogen or helium, is drawn off via connection 16.

The chambers for the buoyant gas can be alternatively sub-divided by substantially horizontal partitions into compartments for the gases that are to be kept separate, and the compartments can be provided with separate connections.

The gas transport hollow bodies can be made without difficulty in diameters of more than lOO m.

In practical operation it can be advantageous to take off by means of the greatest possible gas load with an excess in buoyancy in order to reach great heights rapidly and to expend less propulsion energy. In use of a part of the transported gas in the compressor engines the buoyancy slowly decreases so that at the place of consignment an exact balance is arrived at. For the continuous supply to industry and other gas consumers at the place of consignment there will advantageously be a permanent to-and-fro traffic with a number of gas transport airships.

The cost of transportation with airships drops by about the square of the speed for given distances. It is therefore advisable to provide an optimal estimated speed of about 150 km/hour and to use large ships. With increasing size of the airship the required buoyancy increases only in relationship to the cross section area, while the contents increase cubically, so that a large airship is advantageous. Non-rigid airships have no specific limit for their size because the weight of the envelope increases only linearly with the contents at given velocities.

In addition to the advantages that have been mentioned it should also be noted that the method of the invention offers the most economical of possibilities for geographically flexible gas transportation so that the customary waste of natural sources could be stopped thereby.

What is claimed is:

1. A process for the transportation of impellent gases being lighter than air, e.g., natural gas or hydrogen, from a starting point at which the impellent gas is available to a point of consignment, characterized by the steps of:

substantially completely filling a large capacity, light,

inflatable hollow body with impellent gas at the starting point.

transporting the impellent gas by air with said hollow body to a point of consignment, said hollow body being borne through the air solely by the buoyancy of the impellent gas contained therein, compeltely discharging the impellent gas at the consignment point, while maintaining said hollow body inflated by supplying said hollow body with heated air as said impellent gas is discharged, so that said hollow body will be borne through the air on its return journey to said starting point solely by the buoyancy of said heated air, maintaining said heated air at an elevated temperature during the return journey, and

discharging said heated air after returning to said starting point so that said hollow body can be filled with more impellent gas at said starting point as heated air is discharged.

2. The process according to claim 1 characterized in the combined measure that the difference in buoyancy power between the hollow body being filled with the impellent gas on the delivery journey and the hollow body being filled with the buoyant gas for the return journey is balanced by adequately different amounts of ballast.

3. A process for the transportation of impellent gases being lighter than air, e.g., natural gas or hydrogen, from a starting point at which the impellent gas is available, to a point of consignment, characterized by the steps of:

substantially completely filling a large capacity, light,

inflatable hollow body with impellent gas at the starting point, transporting the impellent gas by air with said hollow body to a point of consignment, said hollow body being borne through the air solely by the buoyancy of the impellent gas contained therein,

completely discharging the impellent gas at the consignment point, while maintaining said hollow body inflated by supplying said hollow body with a mixture of water vapour and heated air as said impellent gas is discharged, so that said hollow body will be borne through the air on its return journey to said starting point solely by the buoyancy of said mixture of water vapour and heated air, and

discharging said mixture of water vapour and heated air after returning to said starting point so that said hollow body can be filled with more impellent gas at said starting point as said mixture of water vapour and heated air is discharged.

4. A process for the transportation of impellent gases being lighter than air, e.g., natural gas or hydrogen, from a starting point at which the impellent gas is available, to a point of consignment, characterized by the steps of:

substantially completely filling a large capacity, light,

inflatable hollow body with impellent gas at the starting point, transporting the impellent gas by air with said hollow body to a point of consignment, said hollow body being borne through the air solely by the buoyancy of the impellent gas contained therein,

completely discharging the impellent gas at the consignment point, while maintaining said hollow body inflated by supplying said hollow body with water vapour as said impellent gas is discharged, so that said hollow body will be borne through the air on its return journey to said starting point solely by the buoyancy of said water vapor,

maintaining said water vapour at an elevated temperature at about C. when transporting the hollow body on said return journey to said starting point, and

discharging said water vapour after returning to said starting point so that said hollow body can be filled with more impellent gas at said starting point as said water vapour is discharged.

5. The process according to claim 4, characterized in that the water vapour is held during the return trip at a temperature that corresponds to its dew point by means of heat exchanges which receive exhaust heat from drive enfiines associated with the propulsion system of the ho ow body. 

1. A process for the transportation of impellent gases being lighter than air, e.g., natural gas or hydrogen, from a starting point at which the impellent gas is available to a point of consignment, characterized by the steps of: substantially completely filling a large capacity, light, inflatable hollow body with impellent gas at the starting point, transporting the impellent gas by air with said hollow body to a point of consignment, said hollow body being borne through the air solely by the buoyancy of the impellent gas contained therein, compeltely discharging the impellent gas at the consignment point, while maintaining said hollow body inflated by supplying said hollow body with heated air as said impellent gas is discharged, so that said hollow body will be borne through the air on its return journey to said starting point solely by the buoyancy of said heated air, maintaining said heated air at an elevated temperature during the return journey, and discharging said heated air after returning to said starting point so that said hollow body can be filled with more impellent gas at said starting point as heated air is discharged.
 2. The process according to claim 1 characterized in the combined measure that the difference in buoyancy power between the hollow body being filled with the impellent gas on the delivery journey and the hollow body being filled with the buoyant gas for the return journey is balanced by adequately different amounts of ballast.
 3. A process for the transportation of impellent gases being lighter than air, e.g., natural gas or hydrogen, from a starting point at which the impellent gas is available, to a point of consignment, characterized by the steps of: substantially completely filling a large capacity, light, inflatable hollow body with impellent gas at the starting point, transporting the impellent gas by air with said hollow body to a point of consignment, said hollow body being borne through the air solely by the buoyancy of the impellent gas contained therein, completely discharging the impellent gas at the consignment point, while maintaining said hollow body inflated by supplying said hollow body with a mixture of water vapour and heated air as said impellent gas is discharged, so that said hollow body will be borne through the air on its return journey to said starting point solely by the buoyancy of said mixture of water vapour and heated air, and discharging said mixture of water vapour and heated air after returning to said starting point so that said hollow body can be filled with more impellent gas at said starting point as said mixture of water vapour and heated air is discharged.
 4. A process for the transportation of impellent gases being lighter than air, e.g., natural gas or hydrogen, from a starting point at which the impellent gas is available, to a point of consignment, characterized by the steps of: substantially completely filling a large capacity, light, inflatable hollow body with impellent gas at the starting point, transporting the impellent gas by air with said hollow body to a point of consignment, said hollow body being borne through the air solely by the buoyancy of the impellent gas contained therein, completely discharging the impellent gas at the consignment point, while maintaining said hollow body inflated by supplying said hollow body with water vapour as said impellent gas is discharged, so that said hollow body will be borne through the air on its return journey to said starting point solely by the buoyancy of said water vapor, maintaining said water vapour at an elevated temperature at about 100*C. when transporting the hollow body on said return journey to said starting point, and discharging said water vapour after returning to said starting point so that said hollow body can be filled with more impellent gas at said starting point as said water vapour is discharged.
 5. The process according to claim 4, characterized in that the water vapour is held during the return trip at a temperature that corresponds to its dew point by means of heat exchanges which receive exhaust heat from drive engines associated with the propulsion system of the hollow body. 